200.1*19.99-20.01*199.7-2.001*1.998
=2001*1.999-2001*1.997-2.001*1.998
=2001*（1.999-1.997）-2.001*1.998
=2001*0.002-2.001*1.998
=2.001*2-2.001*1.998
=2.001*（2-1.998）
=2.001*0.002
=0.004002